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Presented at Short Course III on Exploration for Geothermal Resources,
organized by UNU-GTP and KenGen, at Lake Naivasha, Kenya, October 24 - November 17, 2008.
GEOTHERMAL TRAINING PROGRAMME Kenya Electricity Generating Co., Ltd.
GEOTHERMAL WELL DRILLING
Paul K. Ngugi
Kenya Electricity Generating Company Ltd. (KenGen)
P.O. Box 785, Naivasha
KENYA
pngugi@kengen.co.ke
ABSTRACT
The drilling process complex as it may be rotate about breaking the ground and
lifting the rock cuttings from the resulting hole. The ultimate geothermal drilling
objective is to access the resource for exploitation. However, during resource
development and exploitation, drilling is used to confirm existence of the resource,
obtain data for resource assessment, provide adequate steam fuel for the power
plant and resolve well production complications. Tri-cone tungsten carbide insert
bits are very often used in geothermal drilling. Mobile and conventional land rigs
are predominantly used in the geothermal drilling industry. The rigs are selected to
technically fit the job at the lowest cost possible. The wells are made useful by
casing them. Several casing string are used for each well. They are cemented to
bond them to formation. Large production casing of 13 3/8” casing is increasing
becoming common where large well outputs are encountered and directional
drilling is being employed to target major faults that transmit fluids.
1. OVERVIEW OF THE DRILLING PROCESS
Actual breaking of ground is achieved by use of a rock bit. The bit is rotated under weight. The bit
both crashes and gouges the rock as it rotates. The broken rock pieces arising from the drilling are
lifted from the bore by floating them in a circulating drilling fluid. This process continues until the
well is completed.
2. REASONS FOR DRILLING
The ultimate goal for drilling is to access the resource for exploitation. However, during the resource
development and exploitation drilling serves various purposes.
2.1 Exploration
The very first evaluation of a prospect is achieved through detailed surface reconnaissance. It is aimed
at defining the resource by its key system characteristic namely: existence of a heat source in the form
of hot magmatic body near earth surface, existence of hydrological system, characteristic of the
geological setting and areal extent of the prospect (Figure 1). However, while the surface
measurement and mapping and evaluation of the surface manifestations provide great insight as
regards the resource characteristics and potential, results of the reconnaissance remain inferences and
1
Ngugi 2 Well drilling
are inconclusive. The initial employment of drilling in geothermal prospecting is aimed at providing
proof of exploitable steam and data required for further refining of the conceptual model.
Early Pleistocene 2.2 Appraisal
volcanics
Rift Graben (50-70 km) RAINFALL Striking steam with the
RAINFALL "Recent" first well while is exciting
Geothermal Volcanic Pile Geothermal Well Aberdare
Mau reservoir Ranges opens up doors for more
Ranges
oC questions. Having
dl
C aw confirmed existence of the
o et
ld r
wa ep resource, the next question
ter ocr
pe al is its technical, economic
rc oit
ola n and financial viability.
tio
n Further drilling (appraisal)
is therefore carried out to
Pliocene volcanics and Dikes delineate the resource and
Mozambiquan formation metamorphics NOT TO SCALE establish production well
LITHOSPHERIC MANTLE and reservoir fluids
FIGURE 1: Typical conceptual model of a geothermal system characteristics.
in Kenya
2.3 Production and re-injection
At this stage of development, a decision to construct a plant is already made. The drilling is therefore
to provide sufficient steam to run the plant. Additional wells are drilled for reinjection purpose. One
reinjection well is required for every 4 to 5 production wells.
2.4 Make-up
After commissioning of the power plant, with time the reservoir surfers pressure decline which affects
well productivity. In addition, deposition may occur within the formation around the wells further
reducing wells productivity. With time, therefore further drilling is carried out to replenish the
reduced steam delivery.
2.5 Work-over
Two types of problem may arise during exploitation. Steam depletion in the shallow reservoir may
necessitate deepening of the initial wells or deposition of scales within the well bore may necessitate a
mechanical removal of the scales. These two cases require some form of drilling to accomplish.
3. BITS
3.1 Types of bits
3.1.1 Drag bits
Drag bits is the oldest rotary tool still in use (Figure 2). The cutting blades
are integrally made with the bit body. They are fixed to it and rotate as a
unit with the drill string. The bit is used primarily in soft and gummy
formations
FIGURE 2: Drag bit
Well drilling 3 Ngugi NgugiNgugi
3.1.2 Polycrystalline diamond compacts (PDC) bits
The PDC bits use diamonds inserts embedded on the bit
body (Figure 3). They operate by the diamonds embedding
into the formation and dragged across the face of the rock in
a ploughing action. The diamond bits drills according to the
shear failure mechanism. They are of higher cost but their
long life make them cost economic in certain circumstances.
The PDC bits are used in 5% of the drilling cases in the oil
industry. (Moore 1986). The bits are however hardly used
in geothermal drilling.
3.1.3 Roller cutting bit FIGURE 3: PDC bits
More than 95% of the oilfield footage is drilled today with
tri-cone roller bits (Figure 4). This will form the basis of
our discussions.
3.2 Description-working mechanism
Rotary bits drill the formation using primarily two
principles; 1) rock removal by exceeding its shear strength
and; 2) removal by exceeding the compressive strength
(Adams 1985). The broken rock chips are removed by FIGURE 4: Tri-cone roller bits
scraping or hydraulic cleaning.
Shear failure involves the use of the bit tooth shearing, or cutting, the rock into small pieces so it can
be removed from the area below the rock bit. The simple action of forcing the tooth into the formation
creates some shearing and results in cuttings development. In addition, if the tooth is dragged across
the rock after its insertion, the effectiveness of the shearing action will increase. Shear failure
mechanism requires that the formation exhibit low compressive strength that will allow the insertion
of the tooth. The mechanism is employed while drilling softer formations (Adams 1985).
As the compressive strength or abrasiveness of the formation increases, the shearing – twisting is
reduced. The rock with high compressive strengths generally prevents the insertion of the tooth that
would have initiated the shearing action. In addition, rocks with a high abrasiveness wear the bit tooth
if it is twisted or dragged across the formation face. These types of rocks generally require that a
compressive failure mechanism to be used.
Compressive failure of a rock segment requires that a load be placed on the rock that exceed the
compressive strength for that given rock type. The load must remain, or dwell on the surface long
enough for rock failure to occur. This is the basis for hard–rock drilling characteristics of high bit
weight and low rotary speeds.
3.3 Key design features of the tri-cone bits
Roller cones bits have three components groups; the rolling cones, the bearings and the bit body
(Figures 5, 6 and 7). The body is a forged and welded structure, initially having three pieces, called
the legs, with bearings pins on the lower end of each leg. Each leg also has a nozzle boss and a one
third circular arc-shaped piece at the top. After welding and turning, these three arc-shaped pieces
form the API thread pin connection.
Ngugi 4 Well drilling
Shirttail
Water
boss
FIGURE 5: FIGURE 6: FIGURE 7: Bit body
Bit cone Bit bearing (single leg) with cone and
bearing in place
3.3.1 Cones
Cones bearing axis are designed with an offset from the bit geometric
centre (Figure 8). Ordinarily one would imagine that the bits roll on
the hole bottom surface as the bit is turned. However, due to the
offset, the cones tend to drag across the surface of the formation
resulting in sliding, tearing or shearing, gouging and ripping action by
the teeth on the bottom which help remove chips faster and more
efficiently. For softer-formation, the offset is increased and therefore
increase the ripping action. This means faster drilling with softer
formations. As harder rocks are drilled, the degree of offset for
various bits decreases since compressive failure becomes the primary FIGURE 8: Cone offset
drilling mechanism instead of shearing. Too much offset would cause (Adams 1985)
the bit to wear quickly in hard formations.
3.3.2 Teeth
Two types of teeth exist namely the “mill steel tooth” bit and the tungsten carbide insert bits (TCI).
Under hard, abrasive rocks environment, the milled steel tooth bits are not recommended as they
would wear more rapidly. Tungsten carbide insert bits are more appropriate as they are made of more
wear-resistant materials.
The type of failure mechanism influences bit and
tooth design and bit selection. Soft formations
drilled with shearing actions are drilled most
effectively with long tooth, while harder
formations require more numerous, shorter teeth
(Figure 9). Insert bits use tungsten carbide
buttons pressed into the cone rather than milled,
steel teeth. FIGURE 9: Typical tungsten inserts profiles
3.3.3 Bearings
Roller bits bearings are manufactured in one of three configurations and usually use ball bearing
retainers; unsealed roller bearing, sealed roller bearing and sealed journal bearing
Unsealed bearing, initially grease filled, is exposed to drilling fluids. Failure rate is high due to
increase wear as a results cuttings etc. contacting with the bearing surfaces. Sealed and self
lubricating journal bearing are the premium design both for the steel tooth and TCI bits.
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